1. Field of the Invention
The present invention applies to a device which can measure a viscosity of a molten melt in situ and in real time in a plasma centrifugal furnace.
2. Discussion of the Background
The use of a rotating plasma furnace is being considered for the treatment of nuclear and hazardous waste. Countries throughout the world have now amassed enormous stockpiles of hazardous waste which will take many years and many billions of dollars to remediate.
One process of eliminating such hazardous waste which is currently being assessed by at least the U.S. Environmental Protection Agency (E.P.A.) and the Department of Energy (D.O.E.) is the vitrification of such hazardous waste. In such a process, the hazardous waste material is heated to a very high temperature, which thereby drives off any organic volatile components in the waste material. The remaining material is then left as a molten mass, which forms a glassy structure upon cooling. This vitrification process reduces the volume of the waste, allows hazardous and/or flammable solvents to be eliminated, and can trap the radioactive components in the cooled glass structure, in which case it is extremely resistant to leaching and more suitable for long term disposal. The waste types which can possibly be processed by such a vitrification process include transuranic waste (TRU), mixed waste containing both hazardous and radioactive constituents, and ground soil contaminated with heavy metals. Such waste may also include obsolete weapon parts.
A device which is utilized for such a vitrification process may be a plasma centrifugal furnace. Examples of such a device are disclosed in U.S. Pat. Nos. 4,770,109, 5,005,494 and 5,136,137, and in "A Portable Vitrification System for Waste Treatment", presented at AIChE Mixed Waste Conference, Denver Colo., Aug. 14-17, 1994, Eschenbach et al.
FIG. 1 of this paper to Eschenbach et al is reprinted as FIG. 1 here to show the use of such a plasma arc centrifugal treatment device. Such a device includes a centrifuge 30, in which a slag bath 35 (corresponding to the waste material and also referred to as the molten melt) is provided. This slag bath 35 is heated by a plasma torch. The plasma torch includes a water cooled electrode 5, a plasma gas injection device 10 and a nozzle 15. The electrical arc produced by this plasma torch attaches at the location 40 to an inside of the water cooled electrode 10. The current then flows, by means of an arc formed of ionized gases, past the gas injection 10, through the nozzle 15, and attaches at the location 20 on the surface of the slag bath 35.
In such a type of plasma centrifugal treatment device and process of utilization, the waste material is fed into a rotating primary chamber of centrifuge 30 as slag bath 35 after the drum of the furnace is brought up to a steady operating temperature of, for example, approximately 1380 K. The centrifugal force holds the slag bath 35 to the sides of the rotating drum of centrifuge 30, as the plasma arc torch heats the waste materials by arc termination 20 to a high temperature of, as an example, 1920 K, thereby melting the feed materials and driving off the organic volatiles. Off gases are exhausted to a secondary combustion chamber where appropriate conditions are maintained to ensure complete destruction of the organic contaminants. When the processing of the slag bath 35 of the waste is finished, the rotation rate of the drum of the centrifugal furnace is reduced, and the melt, which is now in the form of a molten glass slag 25, exits by gravitational force through a center hole at a bottom of the drum of centrifuge 30 to a slag collection chamber. At this time, this slag melt 25 solidifies into a low volume nonleachable glass structure.
In such a plasma arc centrifugal furnace employing such a vitrification process, it is necessary to determine the viscosity of the slag bath 35 in situ. If the slag bath 35 is too viscous, then the drain hole may plug during the pouring of the slag bath 35 into the slag collection chamber. If the viscosity of the slag bath 35 is too low, then the melt may corrode/dissolve the refractory lining of the furnace. Furthermore, it is believed that the viscosity of the molten slag bath 35 has a correlation to the quality of the final glass structure. Thus, determining the viscosity of the slag bath 35, before the rotation rate of the drum of the centrifuge 30 is decreased to discharge the slag bath 35 through the drain hole, can provide a useful means to control the final glass structure.
The viscosity of the molten slag bath 35 can be controlled by several methods, depending on the particular process situation. One can control the slag bath 35 viscosity by adjusting the melt temperature (through the amount of heat addition and/or the duration of the heating process) or by varying its composition (through the addition of additives to the slag bath 35). In either case, a quick and reliable method of determining the slag bath 35 viscosity in real time is required, while one adjusts the process variables.
It has been known to measure a viscosity of a liquid by enclosing a liquid between two cylinders (or two disks). In such devices, by keeping one cylinder fixed while the other cylinder is rotated, the viscosity of the liquid can be deduced from a measured torque on either cylinder. Similarly, a rotating bob, stirrer, or paddle wheel can be placed in a stationary container of liquid and the torque required to rotate the bob, etc. can be measured.
However, for the purpose of measuring the viscosity of molten radioactive waste, these known devices suffer from the disadvantage of requiring a portion of the molten melt to be extracted from the treatment system and placed in a separate device for measuring viscosity. Furthermore, the temperature of the measurement device must be maintained at the same temperature as the molten melt (because the viscosity is a strong function of the temperature, and the molten melt will solidify if it cools too much).
Moreover, these methods, and many possible variations upon them, are all based upon a common principle. This principle is that the rotational motion is in a steady state and that the torque required to maintain the rotational motion is also in a steady state, i.e. not varying as a function of time. Further, in these methods the shearing motion necessary to measure the viscosity is set up between one member which rotates and another member which is stationary.